Complex applications in many areas, including scientific computations and business-related systems, are represented as computational workflows composed out of multiple components. There are several approaches that help a user compose these workflows. Some composition systems implement a user-system interactive approach. These systems are useful for expressing user preferences during composition; however, they can be tedious to use if a large number of tasks are needed in the workflow, as composition is done one step at a time, manually. Another approach to workflow composition is full automation, which aims to eliminate unnecessary user interaction during composition. This approach is efficient, but is not ideal if user preferences need to be expressed during composition. Our approach combines the strengths of manual and automatic approaches into mixed-initiative workflow composition. This combined approach uses automated planning techniques, while also incorporating user preferences during composition. Our approach is implemented in a new system, AutoCAT, by combining an interactive workflow editor (Composition Analysis Tool or CAT) and a planner, Prodigy.